It is often desirable to convert an AC signal into a DC signal of equivalent power. Power is frequently characterized by the RMS or Root Mean Square, value. If the input signal is sinusoidal, an equivalent RMS value may be easily determined by filtering the rectified signal into an average value and then by applying a correction factor. However, if the input signal is not sinusoidal, a so-called true RMS measuring system must be used to accurately determine the RMS value of the non-sinusoidal signal.
While such true RMS measuring systems are known in the art and can be used to accurately determine the RMS value of a non-sinusoidal signal, they are not without their problems. For example, the accuracy of such a system may be adversely affected by a coupling circuit that may be interposed between the original signal and the RMS measuring system. An example of such a coupling circuit is an RC (i.e., resistive/capacitive) circuit that is used to filter or remove a DC component (e.g., a DC offset) of the original signal. Such a coupling circuit is commonly referred to as an AC coupling circuit, in that it allows the AC component of the signal to be passed on, while rejecting any DC component of the signal. Signals passing through an AC coupling circuit are often referred to as AC-coupled signals.
Unfortunately, however, AC coupling circuits distort the input signal, particularly at lower frequencies relative to the cut-off frequency of the AC coupling circuit. If not removed or compensated, such distortions will result in an inaccurate determination of the RMS value of the original signal, even though the RMS measuring system itself may be highly accurate. That is, the RMS value determined by the RMS measuring system is based on the distorted signal (i.e., an input signal having an AC coupling error) rather than the original signal.
While systems and methods have been developed which attempt to compensate for the AC coupling errors introduced by AC coupling circuits, they are not capable of compensating for the AC coupling errors in all circumstances. For example, compensation systems have been developed which are effective in compensating for low-frequency distortions imposed by the AC coupling circuits, but only if the input signals are sinusoidal. Another problem that must be addressed relates to the tolerances of the various components of the coupling circuit. That is, variations in the actual values (e.g., resistance and capacitance) of the components of the coupling circuit will mean that a compensation that is suitable for one set of components may be unsuitable for another set of components, even where the actual values of the other set of components are within the specified tolerances. While the selection of high-precision components may reduce this problem, the actual values of the components often change over time. Accordingly, a compensation that is suitable for new components may become unsuitable as the components age. A compensation system should not degrade for components within a tolerance range and should be adjusted for individual component values.